In regenerative heat exchange apparatus, a mass of heat absorbent material commonly comprised of packed element plates is positioned in a hot exhaust gas passageway to absorb heat from the hot gases passing therethrough. After the plates become heated by the gas they are positioned in a passageway being traversed by cool air or other gas where the heated plates give up their heat to the cool air flowing therethrough. After the heat absorbent material has been repeatedly positioned in the hot exhaust gas passage it frequently becomes coated with soot and fly ash thereby rendering it subject to fires and lowering its overall effectiveness of the heat exchanger.
Instruments have been developed, including an infrared ray detector, that may be used to monitor the mass of heat absorbent material to detect an increase in output of infrared rays from the element mass, warn against incipient fires, and when necessary initiate fire control measures within the air preheater. U.S. Pat. Nos. 3,730,259 and 3,861,458 disclose apparatus that is positioned in an air stream facing a heat absorbent matrix therein to detect the infrared rays being emitted by the heated matrix of the heat exchanger. Typically, such instruments include viewing lenses which focus infrared rays given off by the element mass on to the detector.
Since the detection means, including the lens, must be constantly immersed in dust bearing gases, it rapidly becomes clouded or dirty so that it fails to transmit a maximum and true signal to the detector which as a consequence precludes obtaining a rapid and true indication of the temperature or other conditions within the preheater. The lens of such apparatus in continuous use soon becomes clouded so that it results in a loss of viewing efficiency, and then fails to react properly when an emergency occurs. Thus, the effectiveness of the fire detecting apparatus as defined herein is largely dependent upon maintaining a clean viewing lens therefor.
U.S. Pat. No. 4,040,473 discloses a method and apparatus for keeping the viewing lens clean. As disclosed therein, an annular channel surrounds the lens to permit a cleaning fluid to be supplied thereto and flow over the periphery of a lens to remove deposits therefrom.
Although such an application cleans the periphery of the lens well, it fails to remove deposits from the center of the lens. Moreover, it is economically expensive since all fluid lines must be enclosed within the device and they must be provided with expensive insulating features and with pivotal joints. Moreover, the lens holder itself must be made to exacting standards to include suitable duct work therein that results in excessive costs of manufacture and maintenance.